Attention has been focused on a technique for forming a transistor using a semiconductor thin film formed over a substrate having an insulating surface (also referred to as a thin film transistor (TFT)). The transistor is applied to a wide range of electronic devices such as an integrated circuit (IC) or an image display device (display device). A silicon-based semiconductor material is widely known as a material for a semiconductor thin film applicable to a transistor. As another example, an oxide semiconductor has been attracting attention.
For example, a transistor whose active layer includes an amorphous oxide semiconductor containing indium (In), gallium (Ga), and zinc (Zn) is disclosed in Patent Document 1.
Conventional thin film transistors mostly have what is called a planar structure, in which an active layer, an insulating film, an electrode, and the like are stacked over a plane. Owing to advances in manufacturing processes which enable miniaturization of such transistors, channel width is reduced, which causes a reduction in an on-state current. Therefore, in recent years, a transistor having a novel structure which is an alternative to the conventional planar structure has been developed. For example, Patent Document 2 discloses a fin-type transistor in which a polycrystalline silicon film is used as an active layer (referred to as a semiconductor thin film in Patent Document 2).